1. Field of the Invention
The present invention relates to zoom lenses and particularly to a zoom lens that is suitable as an image taking optical system for image pickup apparatuses such as video cameras, security cameras, digital still cameras, broadcast cameras, and silver-halide-film cameras.
2. Description of the Related Art
Image taking optical systems included in image pickup apparatuses, such as video cameras, security cameras, and digital still cameras, including solid-state image pickup devices are desired to be zoom lenses that are of small sizes with wide angles of view and high zoom ratios. Furthermore, such image taking optical systems are desired to be zoom lenses exhibiting high optical performance at all zooming positions. One of known zoom lenses that satisfy such demands is a four-unit zoom lens that includes first to fourth lens units having positive, negative, positive, and positive refractive powers, respectively, in that order from an object side toward an image side.
For example, Japanese Patent Laid-Open No. 8-82743 and U.S. Patent Application Publication No. 2011/0176225 each disclose a rear-focusing four-unit zoom lens in which zooming is performed by moving a second lens unit and a third lens unit, and focusing and correction of image plane variations that may occur during zooming are performed by moving a fourth lens unit.
To realize high optical performance at all zooming positions with a wide angle of view and a high zoom ratio, it is important to appropriately set the type of zooming, the refractive powers of the lens units, the configurations of the lens units, among other parameters. In a rear-focusing four-unit zoom lens, it is important to appropriately set the refractive power, the length of travel during zooming, the magnification, and other parameters of the second lens unit that is used for zooming.
Unless such items are set appropriately, it is difficult to realize high optical performance at all zooming positions while at the same time maintaining the size of the zoom lens small with a wide angle of view and high zoom ratio.